Lithography using self-assembled polymers

ABSTRACT

A method of lithography on a substrate uses a self-assembled polymer (SAP) layer deposited on the substrate, with first and second domains arranged in a pattern across the layer. A planarization layer is formed over the SAP and a development etch applied to substantially remove a portion of the planarization layer over the second domain leaving a cap of the planarization layer substantially covering the first domain. The uncapped second domain is then removed from the surface by a breakthrough etch leaving the capped first domain as a pattern feature on the surface. A transfer etch may then be used to transfer the pattern feature to the substrate using the capped first domain. The capping allows the second domain to be removed, e.g., without excessive loss of lateral feature width for the remaining first domain, even when the difference in etch resistance between the first and second domains is small.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase entry of InternationalPatent Application No. PCT/EP2011/062554, filed Jul. 21, 2011, whichclaims the benefit of U.S. provisional application 61/381,317, which wasfiled on Sep. 9, 2010 and which is incorporated herein in its entiretyby reference.

FIELD

The present invention relates to a method for manufacture of devices bylithography. In particular, the invention relates to a method using aresist layer of self-assembled polymers.

BACKGROUND

In lithography for device manufacture, there is an ongoing desire toreduce the size of features in a lithographic pattern in order toincrease the density of features on a given substrate area. Patterns ofsmaller features having critical dimensions (CD) at nano-scale allow forgreater concentrations of device or circuit structures, yieldingpotential improvements in size reduction and manufacturing costs forelectronic and other devices. In photolithography, the push for smallerfeatures has resulted in the development of technologies such asimmersion lithography and extreme ultraviolet (EUV) lithography.

So-called imprint lithography generally involves the use of a “stamp”(often referred to as an imprint template) to transfer a pattern onto asubstrate. An advantage of imprint lithography is that the resolution ofthe features is not limited by, for example, the emission wavelength ofa radiation source or the numerical aperture of a projection system.Instead, the resolution is mainly limited to the pattern density on theimprint template.

For both photolithography and for imprint lithography, it is desirableto provide high resolution patterning of surfaces, either of an imprinttemplates or of another substrate and a chemical resist or imprintablemedium may be used to achieve this.

The use of self-assembly of block copolymers (BCPs) has been consideredas a potential method for improving the resolution to better values thanthose obtainable by prior lithography methods or as an alternative toelectron beam lithography for preparation of imprint templates.

Self-assemblable block copolymers are compounds useful innanofabrication because they may undergo an order-disorder transition oncooling below a certain temperature (order-disorder transitiontemperature To/d) resulting in phase separation of copolymer blocks ofdifferent chemical nature to form ordered, chemically distinct domainswith dimensions of tens of nanometers or even less than 10 nm. The sizeand shape of the domains may be controlled by manipulating the molecularweight and composition of the different block types of the copolymer.The interface between domains may have a width of the order of 1-5 nmand may be manipulated by modification of the chemical compositions ofthe blocks of the copolymers.

The feasibility of using thin films of block copolymers asself-assembling templates was demonstrated by Chaikin and Register, etal., Science 276, 1401 (1997). Dense arrays of dots and holes withdimensions of 20 nm were transferred from a thin film ofpoly(styrene-block-isoprene) to silicon nitride substrates.

SUMMARY

Block copolymers comprise different blocks, each comprising an identicalmonomer, and arranged side-by side along the polymer chain. Each blockmay contain many monomers of its respective type. So, for instance, anA-B block copolymer may have a plurality of type A monomers in the (oreach) A block and a plurality of type B monomers in the (or each) Bblock. An example of a suitable block copolymer is, for instance, acopolymer having covalently linked blocks of polystyrene (PS) monomer(lyophobic block) and polymethylmethacrylate (PMMA) monomer (lyophilicblock). Other block copolymers with blocks of differinglyophobicity/lyophilicity may be useful. For instance, tri-blockcopolymers (A-B-C) may be useful, as may alternating or periodic blockcopolymers (e.g. [-A-B-A-B-A-B-]_(n) or [-A-B-C-A-B-C]_(m) where n and mare integers). The blocks are connected to each other by covalent linksin a linear or branched fashion (e.g. star or branched configuration).

Block copolymers may form many different phases upon self-assembly,dependent upon the volume fractions of the blocks, degree ofpolymerization within each block type (e.g. number of monomers of eachrespective type within each respective block), the optional use of asolvent and surface interactions. When applied in a thin film, thegeometric confinement may pose additional boundary conditions that maylimit the numbers of phases. In general only spherical (e.g. cubic),cylindrical (e.g. tetragonal or hexagonal) and lamellar phases (i.e.self-assembled phases with cubic, hexagonal or lamellar space-fillingsymmetry) are practically observed in thin films of self-assembled blockcopolymers, and the phase type observed may depend upon the relativevolume fractions of the different polymer blocks.

Suitable block copolymers for use as self-assemblable polymers include,but are not limited to, poly(styrene-b-methylmethacrylate),poly(styrene-b-2-vinylpyrididne), poly(styrene-b-butadiene),poly(styrene-b-ferrocenyldimethylsilane), poly(styrene-b-ethylenoxide),poly(ethyleeneoxide-b-isoprene). The symbol “b” signifies “block”Although these are di-block copolymers as examples, self-assembly mayalso or instead employ tri-block, tetrablock or other multi-blockcopolymers.

The self-assembled block copolymer phases may orient with symmetry axesparallel or perpendicular to the substrate and lamellar and cylindricalphases are most interesting for lithography applications, as they mayform line and space patterns and hole arrays, respectively, and mayprovide good contrast when one of the domain types is subsequentlyetched.

Two methods used to guide self-assembly of block copolymers ontosurfaces are graphoepitaxy and chemical pre-patterning. In thegraphoepitaxy method, self-organization of block copolymers is guided bytopological pre-patterning of the substrate. Self-aligned blockcopolymers can form, for example, parallel linear patterns with adjacentlines of the different polymer block domains in the trenches defined bythe patterned substrate. For instance if the block copolymer is adi-block copolymer with A and B blocks within the polymer chain, where Ais lyophilic (e.g., hydrophilic) and B is lyophobic (e.g., hydrophobic)in nature, the A blocks may assemble into a domain formed adjacent to aside-wall of a trench if the side-wall is also lyophilic in nature.Resolution may be improved over the resolution of the patternedsubstrate by the block copolymer pattern subdividing the spacing of apre-pattern on the substrate.

In the chemical pre-patterning method, the self-assembly of blockcopolymer domains is guided by a chemical pattern on the substrate.Chemical affinity between the chemical pattern and at least one of thetypes of copolymer blocks within the polymer chain may result in theprecise placement of one of the domain types onto a corresponding regionof the chemical pattern on the substrate. For instance if the blockcopolymer is a di-block copolymer with A and B blocks, where A islyophilic and B is lyophobic in nature, and the chemical patterncomprises a lyophobic region on a lyophilic surface, the B domain maypreferentially assemble onto the lyophobic region. As with thegraphoepitaxy method of alignment, the resolution may be improved overthe resolution of the patterned substrate by the block copolymer patternsubdividing the spacing of one or more pre-patterned features on thesubstrate (so-called density multiplication). Chemical pre-patterning isnot limited to a linear pre-pattern; for instance the pre-pattern may bein the form of a 2-D array of dots suitable as a pattern for use with acylindrical phase-forming block copolymer. Graphoepitaxy and chemicalpre-patterning may be used, for instance, to guide the self-organizationof lamellar or cylindrical phases.

In a typical process to implement the use of block copolymerself-assembly in nanofabrication, a substrate may be modified with anorientation control layer, or primer layer, to induce the preferredorientation of the self-assembly pattern in relation to the substrate.For some block copolymers used in a self-assemblable polymer layer,there may be a preferential interaction between one of the blocks andthe substrate surface that may result in orientation. For instance, fora polystyrene(PS)-b-PMMA block copolymer, the PMMA block willpreferentially wet (i.e. have a high chemical affinity with) an oxidesurface and this may be used to induce the self-assembled pattern to lieoriented parallel to the plane of the surface. Perpendicular orientationmay be induced, for instance, by rendering the substrate surface neutralto both blocks, in other words having a similar chemical affinity foreach block, such that both blocks wet the surface in a similar manner. Aneutral surface may be created by use of random copolymer brushes whichare covalently linked to the substrate by reaction of a hydroxylterminal group, or some other reactive end group, to oxide at thesubstrate surface. Alternatively or additionally, a crosslinkable randomcopolymer or an appropriate silane can be used to render a surfaceneutral by acting as an intermediate layer between the substrate surfaceand the layer of self-assemblable polymer. An intermediate layer betweenthe substrate and the layer of self-assemblable polymer, onto which theself-assemblable polymer layer is directly deposited, is referred tohereinafter as a primer layer. The primer layer may be provided with oneor more gaps to permit one of the block types of the self-assemblablepolymer layer to come into direct contact with the substrate below theprimer layer. This may be useful for anchoring or aligning a domain of aparticular block type of the self-assemblable polymer layer to thesubstrate.

The substrate may be further provided with a topological or chemicalpre-pattern as set out above to guide the self-assembly pattern. A thinlayer of self-assemblable block copolymer may be deposited on thesubstrate, e.g., onto an intermediate neutral or primer layer as set outabove. A suitable method for deposition is spin coating as it is capableof providing a well defined, uniform, thin layer. A suitable layerthickness for a deposited block copolymer film is approximately 10 to100 nm. Following deposition of the block copolymer film, the film maystill be disordered or only partially ordered and additional steps maybe needed to promote and/or complete self-assembly.

The block copolymers are self-assembled into a highly ordered regularpattern. The self-assembly process typically occurs most readily at atemperature above the glass-transition temperature and below theorder-disorder temperature for the block copolymer. This stage isreferred to as ordering, and is generally achieved by uniform heating.The self-assembly process may nucleate at multiple points in the blockcopolymer film and this may result in the formation of defects.

Defects formed during ordering as set out above may be partly removed byannealing. Defects such as disclinations (which are line defects inwhich rotational symmetry is violated, e.g. where there is a defect inthe orientation of a director) may be annihilated by pairing with otherdefects or disclinations of opposite sign. Chain mobility of the blockcopolymer may be a significant factor for determining defect migrationand annihilation and so annealing may be carried out at a temperaturewhere chain mobility is high but the self-assembled ordered pattern isnot lost. This implies a temperature up to a few ° C. above or below theorder/disorder temperature To/d for the polymer.

Unfortunately, some isolated defects are hard to annihilate. They have ahigh immobility which may be attributable to high energy input requiredto restructure the ordered surroundings of such defects.

Ordering and defect annihilation may be combined into a single annealingprocess or a plurality of processes may be used in order to provide alayer of self-assembled block copolymer, having an ordered pattern ofdomains of differing chemical type (of different block types), for useas a resist layer for lithography.

Self-assembly of block copolymers is a process where the assembly ofmany small components (the block copolymers) results in the formation oflarger more complex structures (the nanometer sized features in theself-assembled pattern, referred to as domains in this specification).Defects arise naturally from the physics controlling the self-assemblyof the polymers. Self-assembly is driven by the differences ininteractions (i.e. differences in mutual chemical affinity) between A/A,B/B and A/B (or B/A) block pairs of an A-B block copolymer, with thedriving force for phase separation described by Flory-Huggins theory forthe system under consideration.

For block copolymers which undergo self-assembly, the block copolymerwill exhibit an order-disorder temperature To/d. To/d may be measured byany suitable technique for assessing the ordered/disordered state of thepolymer, such as differential scanning calorimetry (DSC). If layerformation takes place below this temperature, the molecules will bedriven to self-assemble. Above the temperature To/d, a disordered layerwill be formed with the entropy contribution from disordered A/B domainsoutweighing the enthalpy contribution arising from favorableinteractions between neighboring A-A and B-B block pairs in the layer.

The block copolymer may also exhibit a glass transition temperature Tgbelow which the polymer is effectively immobilized and above which thecopolymer molecules may still reorient within a layer relative toneighboring copolymer molecules. The glass transition temperature issuitably measured by differential scanning calorimetry (DSC).

In the following discussion, and generally throughout the specification,the self-assemblable polymer is treated as having two domain types(based on polymer blocks A and B respectively) forming the orderedpattern. The same description applies for self-assemblable polymers withthree or more different domain types forming the ordered pattern,modified to take into account the additional domain types.

In order to transfer a pattern from the self-assembled polymer layer, afirst domain type will typically be removed by so-called breakthroughetching to provide a pattern of a second domain type on the surface ofthe substrate with the substrate laid bare between the pattern featuresof the second domain type. It is to be noted that the term substrate asused in this specification may include a substrate, such as asemiconductor substrate, having one or more assist layers on the surfaceto which lithography is to be applied, and these one or more layers aretreated as part of the substrate.

Following the breakthrough etching, the pattern may be transferred byso-called transfer etching using an etching means (e.g., an etchant)which is resisted by the second domain type and so forms recesses in thesubstrate surface where the surface has been laid bare.

Typically, for self-assemblable block copolymers, the difference in etchresistance between the different domain types is small. This may bereferred to as low etch resistance contrast. For instance, for PS(polystyrene) and PMMA (polymethylmethacrylate) blocks, the etchresistance contrast is 1:1.5 and 1:2.9 for an oxygen plasma etch and foran argon plasma etch respectively (i.e. for every 1 nanometer of PSremoved, 1.5 nanometers of PMMA is removed).

As an alternative to plasma etching, selective photolytic removal of adomain type is possible. For instance a PMMA domain may be removed byphotolytic etching to leave the substrate laid bare between islands orpattern features of polystyrene.

Because of the thickness of a typical self-assembled polymer layer(typically about 1.5 times the pitch of the ordered pattern), andbecause of the relatively poor etch contrast which is inherent toself-assembled block copolymers, the breakthrough etch, in addition tosubstantially completely removing one domain type, may lead to reductionin a lateral dimension of the one or more remaining pattern features ofthe second domain type. It is also the case that a domain may exhibitanisotropic behavior when subject to breakthrough etching. This isundesirable as it may lead to imprecise control of critical dimension(CD) for the patterned substrate formed after a transfer etch.

The thermodynamics of polymer self-assembly is predominantly governed bythe immiscibility (i.e. lack of mutual chemical affinity) of thechemical species making up the different blocks as described inFlory-Huggins theory. Whether or not a polymer will exhibitself-assembly into nanoscale patterns is characterized by the product_(X)N, in which _(X) is the Flory-Huggins interaction parameter and N isthe total degree of polymerization.

Mean field theory calculations indicate that an order-disordertransition should occur for _(X)N=10.5 for symmetrical block copolymers,which implies that the minimum total degree of polymerization needed togive phase segregation for a class of block copolymers is dependent uponthe Flory-Huggins parameter for this block copolymer at the annealingtemperature following the relationship N=10.5/_(X). It should beunderstood that the value of 10.5 is not to be taken as a threshold, butrather it indicates an approximate theoretical region above or belowwhich ordering or disordering may take place. When _(X)N is less thanabout 10.5 the block copolymer will not generally exhibit self-assemblybehavior and the resulting polymer film will be in a disordered, highentropy state. For _(X)N greater than about 10.5, the block copolymermay show strong segregation of the block domains and the copolymer mayself-assemble into regular patterns with sharp interfaces betweenadjacent domains of differing block types.

For block copolymers with _(X)N greater than 10.5, self-assembledpattern formation occurs when a thin film sample is brought to atemperature sufficiently above the glass transition temperature of thecopolymer to provide reasonable polymer mobility, but below To/d for thecopolymer, to allow nano-phase separation and pattern formation.

The degree of polymerization N is directly related to the minimumcharacteristic domain length scale, L₀, attainable for a specific blockcopolymer. Typically L₀=N^(δ), where δ can vary from 0.5 to 1.0depending on whether the polymer exhibits strong or weak segregation. L₀gives an indication of the smallest feature sizes derivable for theself-assembled polymer.

The line edge roughness, LER, and line width roughness, LWR, ofself-assembled features for a self-assemblable block copolymer aregoverned by polydispersity of the block copolymers and by theinterfacial width between polymer blocks at the domain boundariesbetween adjacent domains of differing block type. The interfacial widthis given by the relationship Δ_(∞)=2a/√6_(X).

The LWR is typically related to LER for self-assembled polymers by therelationship LWR=√2 LER.

Hence, the LWR and LER are inversely proportional to the Flory-Hugginsparameter _(X), and so it is desirable that _(X) is as large as possiblewhen the self-assembled polymer is fixed in its structure (for instancewhen the self-assembled polymer is solidified by reducing itstemperature to a temperature below the glass transition temperature Tgfor the polymer) so that LWR/LER are as small as possible.

Limited pattern persistence length and high defect densities may limitthe usefulness of a self-assemblable polymer, such as block copolymers,for use in the semiconductor industry. Both the pattern persistencelength of an ordered polymer layer and its defect density may beinfluenced by the value of the Flory-Huggins parameter at thetemperature at which the ordered polymer layer is formed or annealed(i.e. at a temperature at or around To/d for the polymer). Persistencelength is a measure of the ability to maintain coherence of periodicityover long distances.

A high value for the Flory Huggins parameter _(X) at the annealingtemperature strongly inhibits inter-diffusion of polymer blocks ofdiffering types, and so hinders the processes leading to defectannihilation and pairing, but a high value of _(X) is desirable to givelow line edge and line width roughness (LER/LWR) for the assembled,ordered pattern. In practice, the requirement to reach thermodynamicequilibrium by providing high polymer chain mobility and interdiffusionof polymer blocks means that the chemical incompatibility of the blocksshould not be too great. If the blocks are relatively chemicallycompatible, but sufficiently incompatible for self-assembly to occur atlow To/d, self-assembled, highly ordered patterns will form but this islikely to be at the expense of relatively poor etch resistance contrastbetween the different domain types. For chemically similar species, thedifference in etch resistance for each species to the same etch islikely to be small.

The Flory-Huggins parameter for PS-b-PMMA(polystyrene-b-polymethylmethacrylate) block copolymer allows for thegeneration of features down to 22 nm, but for smaller features otherblock copolymers are used, such as polypentafluorostyrene(PPFS)-b-PMMA,PS-PVP(polyvinylpyrrolidone), PS-PFI (fluorinated polyisoprene), or PEO(poly ethyleneoxide)-IP(polyisoprene).

As there may be low etch resistance contrast between two domain typesbased on differing polymer blocks, the etch contrast may not besufficient to allow for a proper transfer of the features into theunderlying substrate. It is noted that the height of the domains for theself-assembled layer is typically 1.5 times the pitch, so for example a50 nm height for 16 nm half-pitch node (32 nm pitch).

The selection of self-assemblable polymers to achieve optimalself-assembly results (such as low defectivity, low LWR/LER, etc.) for acertain node is challenging in its own right. To further include aselection requirement for sufficient etch resistance contrast betweenthe domain types formed from the different blocks, so that accuratetransfer of the pattern to the substrate is achievable, is an additionalconstraint that would significantly limit freedom in material choice forthe blocks. Hence there is a need to enhance etch resistance contrastfor the domain types by another means.

According, it is desirable, for example, to provide a method for using aself-assembled layer of block copolymer as a resist layer suitable foruse in device lithography which addresses or overcomes, for example, oneor more of the problems in the art. It is desirable, for example, toprovide a method which improves the accuracy of transfer, by etching,from a self-assembled pattern to an underlying substrate. It isdesirable, for example, to improve the accuracy of transfer from aself-assembled polymer layer having at least two domain types with lowetch resistance contrast between the two domain types.

Throughout this specification, the term “comprising” or “comprises”means including the component(s) specified but not to the exclusion thepresence of others. The term “consisting essentially of” or “consistsessentially of” means including the components specified but excludesother components except for materials present as impurities, unavoidablematerials present as a result of processes used to provide thecomponents, and components added for a purpose other than achieving thetechnical effect of the invention. Typically, a composition consistingessentially of a set of components will comprise less than 5% by weight,typically less than 1% by weight of non-specified components.

Whenever appropriate, the use of the term “comprises” or “comprising”may also be taken to include the meaning “consists essentially of” or“consisting essentially of”.

According to an aspect, there is provided a method of lithography on asurface of a substrate having a self-assembled polymer layer thereoncomprising first and second domains arranged in a pattern across thelayer, the method comprising:

forming a planarization layer over the layer of self-assembled polymer,the planarization layer having a first portion over the first domain anda second portion over the second domain;

applying a development etch to the planarization layer to substantiallyremove the second portion leaving at least part of the first portion asa cap substantially covering the first domain to form capped firstdomain;

substantially removing the second domain from the surface, leaving thecapped first domain as a pattern feature on the surface; and

transferring the pattern feature to the substrate using the capped firstdomain as an etch resist.

According to an aspect, there is provided a method for transferring apattern feature from a self-assembled polymer layer comprising first andsecond domains arranged in a pattern across the layer on a substrate, tothe substrate, the method comprising:

applying a first etch to the self-assembled polymer layer such that thefirst domain is etched to a first thickness less than a second thicknessof the second domain;

forming a planarization layer over the layer of self-assembled polymer,the planarization layer having a first portion over the first domain anda second portion over the second domain;

applying a development etch to the planarization layer to substantiallyremove the second portion leaving at least part of the first portion asa cap substantially covering the first domain to form capped firstdomain;

substantially removing the second domain from the surface, leaving thecapped first domain as a pattern feature on the surface; and

transferring the pattern feature to the substrate using the capped firstdomain as an etch resist.

According to an aspect, there is provided a method of forming a patternfeature on a substrate from a self-assembled polymer layer thereon, theself-assembled polymer layer comprising first and second domainsarranged in a pattern across the layer, the method comprising:

forming a planarization layer over the layer of self-assembled polymer,the planarization layer having a first portion over the first domain anda second portion over the second domain;

applying a development etch to the planarization layer to substantiallyremove the second portion leaving at least part of the first portion asa cap substantially covering the first domain to form capped firstdomain; and

substantially removing the second domain from the surface, leaving thecapped first domain as a pattern feature on the surface.

The following features are applicable to all the various aspects of theinvention where appropriate. When suitable, combinations of thefollowing features may be employed as part of the methods andcompositions described herein, for instance as set out in the claims.The methods and compositions described herein are particularly suitablefor use in device lithography. For instance the methods and compositionsdescribed herein may be used for treatment or formation of a resistlayer of self-assembled polymer for use in patterning a device substratedirectly or for use in patterning an imprint template for use in imprintlithography.

The self-assemblable polymer may be a block copolymer as set outhereinbefore comprising at least two different block types which areself-assemblable into an ordered polymer layer having the differentblock types associated into first and second domain types. The blockcopolymer may be a di-block copolymer or a tri-block or a multi-blockcopolymer. Alternating or periodic block copolymers may be used as theself-assemblable polymer. Although only two domain types may bementioned in some of the following aspects and examples, an embodimentof the invention is also applicable to self-assemblable polymers withthree or more different domain types.

In an embodiment, the self-assemblable polymer is a block copolymercomprising one or more first blocks of first monomer and one or moresecond blocks of second monomer.

By chemical affinity, in this specification, is meant the tendency oftwo differing chemical species to associate together. For instancechemical species which are hydrophilic in nature have a high chemicalaffinity for water whereas hydrophobic compounds have a low chemicalaffinity for water but a high chemical affinity for alkanes. Chemicalspecies which are polar in nature have a high chemical affinity forother polar compounds and for water whereas apolar, non-polar orhydrophobic compounds have a low chemical affinity for water and polarspecies but may exhibit high chemical affinity for other non-polarspecies such as alkanes or the like. The chemical affinity is related tothe free energy associated with an interface between two chemicalspecies: if the interfacial free energy is high, then the two specieshave a low chemical affinity for each other whereas if the interfacialfree energy is low, then the two species have a high chemical affinityfor each other.

By “chemical species” in this specification is meant either a chemicalcompound such as a molecule, oligomer or polymer, or, in the case of anamphiphilic molecule (i.e. a molecule having at least two interconnectedmoieties having differing chemical affinities), the term “chemicalspecies” may refer to the different moieties of such molecules. Forinstance, in the case of a di-block copolymer, the two different polymerblocks making up the block copolymer molecule are considered as twodifferent chemical species having differing chemical affinities.

Suitably, the first domain has a first thickness less than a secondthickness of the second domain. This could arise naturally from the selfassembly process for instance, or may desirably be as a result ofapplication of a treatment to the self-assembled polymer layer. Forinstance, the treatment may be a first etch, or may be a photolyticprocess (e.g. UV irradiation of PMMA blocks may lead to their erosion).Another suitable process may be surface reconstruction using a solvent,such as set out in Gowd et. al. Nanotechnology 20 (2009) 415302.

The method may comprise applying a first etch to the self-assembledpolymer layer such that the first domain is etched to a first thicknessless than a second thickness of the second domain prior to forming theplanarization layer.

The first etch may be, for instance, a chemical etch (e.g. byapplication of an etch solution) or may be a plasma etch. In anembodiment, the first etch may be a directional (anisotropic) etch suchas a directional plasma (i.e. reactive ion) etch, or a photolytic etch.When the first etch is a plasma or reactive ion etch, it may be, forinstance, an oxygen plasma etch or an argon plasma etch. A reactive ionetch uses a plasma combined with an electric field to accelerate ionsfrom the plasma into the substrate. This is a directional (i.e.anisotropic) etch.

The first etch may be applied as a uniform etch to the layer ofself-assemblable polymer, with the difference in thickness between thefirst and second domains arising from the differing resistances toetching of the first and second domains. This etch resistance contrastwill typically arise from the differing chemical affinities of thechemical species making up the first and second domains respectively,with the first domain being removed more rapidly than the second domainby the first etch such that the first domain will have a smallerthickness than the second domain after the first etch if both first andsecond domains are initially of the same thickness in the self-assembledpolymer layer.

As already explained herein, the etch resistance contrast for first andsecond domains may be relatively low, meaning that the etching rates forthe two domain types are not greatly different from each other, so thatthe application of the first etch to the self-assemblable polymer mayonly give rise to a small difference in thickness between first andsecond domains. However, even a small difference in thickness may besufficient for a cap of planarization layer to be left remaining overthe first domain, after formation of the planarization layer andapplication of a development etch to remove the planarization layer overthe second domain.

As with the first etch, the development etch may be uniformly applied tothe planarization layer. For the reasons set out above, theplanarization layer will have a greater depth over the first domain thanover the second domain because of the differences in thickness betweenthe remaining first and second domains after the first etch. Even thoughthe planarization layer may not have a completely planar or flat uppersurface, and may partially follow the topography of the self-assembledpolymer layer on which it is deposited, provided that the planarizationlayer is thicker over the first domain than it is over the seconddomain, then a uniformly applied development etch, applied for asuitable time, should be able to substantially remove the planarizationlayer over the second domain leaving a cap of the planarization layer inplace over the first domain.

Typically a Si-containing material will be used as a planarizationlayer, in which case a fluorine based etch (e.g. CF₄ or CHF₃ plasmaetch) may be used for the development etch.

The second domain may then be substantially removed from the surface,leaving the capped first domain as a pattern feature on the surface.This may suitably be achieved by application of a breakthrough etch tothe substrate and layers thereon. The breakthrough etch is suitably anetch which is capable of removing the second domain substantially orcompletely to lay bare the surface of the substrate in the locationpreviously occupied by the second domain, while leaving the first domainin place, covered by a protective cap of the planarization layer, whichis substantially resistant to removal by the breakthrough etch. In atypical case the breakthrough etch may be an oxygen plasma etch, whichetches the non-Si containing domain, but to which the Si-containingplanarization layer is resistant. Alternatively, the removal of thesecond domain may employ the same etching method as the first etch, withthe proviso that the material of the planarization layer, forming thecap, should be sufficiently resistant to the etching to remain in placefollowing removal of the second domain. In other words, etching of thesecond domain using the same etching as the first etch should be muchfaster than the etching rate for the planarization material.

As a result, the desired pattern feature may be transferred to thesubstrate using the remaining capped first domain as an etch resist whena transfer etch is carried out.

A suitable transfer etch will depend upon the substrate. For a siliconsubstrate, a fluorine based etch (e.g. CF₄ or CHF₃ plasma etch) may beemployed.

In an embodiment, the development etch is applied for as short a timenecessary to provide an adequate difference in thicknesses between thefirst and second domains without leading to excessive removal of thedomains. This is to minimize any lateral erosion of the domains duringthe first etch.

The difference in thickness may, for instance, be 5 nm or more. Forexample, the first thickness may be 5 nm or more. The second thicknessis 5 nm, or more, greater than the first thickness.

In this specification, when reference is made to the thickness of afeature, the thickness is suitably measured by an appropriate meansalong an axis normal to the substrate surface and passing through thecentroid of the feature. Thickness may suitably be measured by atechnique such as interferometry or assessed through knowledge of etchrates.

Planarization layers are known in the art of device lithography and areused to provide a substantially level or flat topography over anunderlying, uneven topography. A suitable planarization layer is of amaterial which will flow into one or more recesses in the underlyingtopography while self-levelling on its upper surface. Typically theplanarization layer will be a flowable liquid when initially applied andsolidified after levelling. Typically, the planarization layer will be acontinuous planarization layer over the self-assembled polymer layer.

The planarization layer may be applied, for instance, by spin-coating aliquid planarization composition onto the self-assembled polymer layerand solidifying the liquid planarization composition to provide theplanarization layer. Other suitable methods for application or formationof a planarization layer include dip-coating, blade-coating or chemicalor vapor deposition.

Solidification may be achieved, for example, by cooling a liquidplanarization composition which is a melt below its solidificationtemperature, or by evaporation of solvent from a liquid planarizationcomposition which is a solution.

The planarization layer may suitably be of a planarization materialselected from the group consisting of silicon-containing acrylatepolymer, hydrogen silsesquioxane and polydimethylsiloxane. The presenceof silicon in the planarization layer may be effective in making theplanarization layer resistant to certain etch compositions or etchingmethods.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the invention will be described with referenceto the accompanying figures, in which:

FIGS. 1A to 1C schematically depict directed self-assembly of A-B blockcopolymers onto a substrate by graphoepitaxy and formation of reliefpatterns by selective etching of one domain;

FIGS. 2A to 2C schematically depict directed self-assembly of A-B blockcopolymers onto a substrate by chemical pre-patterning and formation ofa relief pattern by selective etching of one domain;

FIGS. 3A and 3B schematically depict a method for transferring a patternfeature from a self-assembled polymer layer to a substrate; and

FIGS. 4A to 4E schematically depict an embodiment of a method fortransferring a pattern feature from a self-assembled polymer layer to asubstrate.

DETAILED DESCRIPTION

FIG. 1A shows a substrate 1 with a trench 2 formed therein bounded byone or more side walls 3 and a bottom surface 4. In FIG. 1B, aself-assemblable A-B block copolymer with lyophilic A blocks andlyophobic B blocks has been deposited into the trench to form a layer 5with alternating stripes of A and B domains which have deposited as alamellar phase separated into discrete micro-separated periodic domainsduring deposition of the block copolymer. This is referred to asgraphoepitaxy. The type A domains have nucleated adjacent to the sidewall 3, which is also lyophilic. In FIG. 1C, the type A domains havebeen removed by selective chemical etching, leaving the type B domainsto form a relief pattern in the trench where they may serve as atemplate for subsequent patterning of the bottom surface 4, for instanceby further chemical etching. Selective removal may also be achieved, forinstance, by selective photodegradation or photo-cleavage of a linkingagent between blocks of the copolymer and subsequent solubilization ofone of the blocks. The pitch or wavelength of the self-assembled polymerstructure 5 and the width of the trench 4 are arranged so that a numberof alternating stripes of domains can fit into the trench within thesidewall with a type A domain against each side wall.

FIG. 2A shows a substrate 10 with a chemical pattern in the form ofpinning stripes 11 which have been chemically formed on the surface 13to provide regions with a higher affinity for the type A blocks of thepolymer. In FIG. 2B, a self-assemblable A-B block copolymer withlyophilic A blocks and lyophobic B blocks has been deposited onto thesurface 13 of substrate 10 to form a lamellar phase layer 12 withalternating stripes of A and B domains which have phase separated intodiscrete micro-separated periodic domains during deposition of the blockcopolymer. This is referred to as chemical pre-patterning. The type Alyophilic domains have nucleated atop the pinning stripes 11, which arealso lyophilic. In FIG. 1C, the type A domains have been removed byselective chemical etching, leaving the type B domains to form a reliefpattern on the surface 13 where they may serve as a template forsubsequent patterning of surface 13, for instance by further chemicaletching. The pitch or wavelength of the self-assembled polymer structure12 and the spacing of the pinning stripes 11 are arranged so that anumber of alternating stripes of domains can fit between the pinningstripes 11 with a type A domain atop each pinning stripe 11.

FIGS. 3A and 3B schematically depict a method for transferring a patternfeature from a self-assembled polymer layer 21 to a substrate 20. InFIG. 3A, the substrate 20 has a self-assembled polymer layer 21 on itssurface, the self-assembled polymer layer 21 having first 22 and second23 domains forming an alternating pattern across the layer 21. For theexample shown, the self-assemblable polymer is a block copolymerpolystyrene(PS)-block-polymethylmethacrylate(PMMA), with PMMA as firstdomain 22 and PS as second domain 23. The etch resistance contrast isabout 1:1.5 (PS:PMMA) for an oxygen plasma etch.

FIG. 3B shows the structure after application of an oxygen plasma etchto substantially remove the first domain 22 of PMMA. The poor etchresistance contrast means that a substantial reduction in the lateralextent of the remaining islands 24 of second domain has also been causedby the oxygen plasma etch. Hence the remaining pattern features formedby the islands 24, when subsequently transferred into the substrate by atransfer etch, using the islands 24 as resist, will not provide anaccurate transfer of the features of the original self-assembled polymerlayer.

FIGS. 4A to 4E schematically depict a method according to an embodimentof the invention for forming pattern features from a self-assembledpolymer layer 21 on a substrate 20 ready for subsequent transfer intothe surface of the substrate 20 by transfer etching. In FIG. 4A, thesubstrate 20 has a self-assembled polymer layer 21 on its surface, thelayer 21 having first 22 and second 23 domains forming an alternatingpattern across the layer 21. As for the prior example depicted in FIG.3A, the self-assemblable polymer is a block copolymerpolystyrene(PS)-block-polymethylmethacrylate(PMMA), with PMMA as firstdomain 22 and PS as second domain 23.

FIG. 4B shows the structure following a short application of an oxygenplasma etch to remove part of the first domain 22 of PMMA. The poor etchresistance contrast means some of the second domain 23 will also beremoved during the etch, but the lateral extent of the second domainwill not be significantly reduced because a short etch time is adequateto provide a difference in thickness between T1, the thickness of thefirst domain, and T2, the thickness of the second domain.

In FIG. 4C, following the first etch of the layer 21 to provide recessesof depth (T2−T1) in its upper surface over the first domain 22, aplanarization layer 25 is deposited over the layer 21 as a liquid melt,filling the recesses over the first domain 22 and covering the tops ofthe second domain 23, and subsequently solidified by cooling. It will beunderstood that although FIG. 4C shows the planarization layer 25 ashaving a planar upper surface, in practice, this may exhibit undulationsarising from incomplete leveling of the planarization layer 25 prior toits solidification. However, the thickness of the planarization layer 25measured over the first domain 22 should be greater than its thicknessmeasured over the second domain 23.

FIG. 4D shows the structure following uniform application of adevelopment etch using a CHF₃ or CF₄ plasma etch whereby theplanarization layer 25 has been partially removed so that the tops ofthe second domain 23 have again been laid bare, but leaving caps 26 ofplanarization layer 25 in the recesses and covering the tops of thefirst domain 22 to form capped first domain 27. End point detectionusing e.g. optical detection or mass-spectrometry in situ may be used todetermine the correct moment to terminate the first etch step. Forinstance, a step change in Si content of the detected vapor from theplasma will indicate that the tops of domain 23 have been laid bare.

In FIG. 4E, the structure is shown after a breakthrough etch, once againan oxygen plasma etch, has been uniformly applied to the upper surface.The caps 26 are of a material highly resistant to the breakthrough etchand protect the first domain 22 from being eroded, whereas the uncoveredsecond domain 23 is removed by the breakthrough etch to lay bare thesubstrate surface where it stood leaving pillars of capped first domain27 with caps 26 still in place.

Hence the remaining pattern features formed by the pillars of the cappedfirst domain 27, when subsequently transferred into the substrate by atransfer etch, using the capped pillars 22,26 as resist, will provide amore accurate transfer of the features of the original self-assembledpolymer layer 21 than the method as schematically shown in FIGS. 3A and3B. However, it is to be noted that the features are effectivelyreversed (i.e. a negative image) when the method of FIG. 4 is used asopposed to the method of FIG. 3.

The described and illustrated embodiment is to be considered asillustrative and not restrictive in character, it being understood thatonly a preferred embodiment have been shown and described and that allchanges and modifications that come within the scope of the invention asdefined in the claims are desired to be protected. For instance, theremay be a substrate planarization layer and/or a bottom anti-reflectioncoating layer interposed between the self-assembled polymer layer andthe substrate. For instance, the first etch may not be necessary if theself-assembled polymer layer is such that the thicknesses T1 and T2differ for the first and second domains immediately upon assembly.

An embodiment of the invention permits etch resistance contrast to beachieved by use of a cap of planarization layer rather than byconstraining selection of the chemical species used for the first andsecond domains of the self-assemblable polymer. Hence, the polymerchemistry may be optimized for characteristics other than etch resistcontrast, using an embodiment of the invention to compensate for lowetch resist contrast by reducing undesired lateral erosion of patternfeatures. Much greater etch resistance contrast may be obtainable thanwould conventionally be attainable using a self-assembled polymer layer,thus providing a substantial benefit when transferring a pattern from aself-assembled polymer layer to a substrate.

An embodiment of the present invention relates to lithography methods.The methods may be used for the manufacture of devices, such aselectronic devices and integrated circuits or other applications, suchas the manufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, flat-panel displays,liquid-crystal displays (LCDs), thin film magnetic heads, organic lightemitting diodes, etc. An embodiment of the invention is also of use tocreate regular nanostructures on a surface for use in the fabrication ofbit-patterned media or discrete track media for magnetic storage devices(e.g. for hard drives).

In particular, an embodiment of the invention is of use for highresolution lithography, where features patterned onto a substrate have afeature width or critical dimension of about 1 μm or less, typically 100nm or less or even 10 nm or less.

Lithography may involve applying several patterns onto a substrate, thepatterns being stacked on top of one another such that together theyform a device such as an integrated circuit. Alignment of each patternwith a previously provided pattern is an important consideration. Ifpatterns are not aligned with each other sufficiently accurately, thenthis may result in some electrical connections between layers not beingmade. This, in turn, may cause a device to be non-functional.Lithographic apparatus therefore usually includes an alignmentapparatus, which may be used to align each pattern with a previouslyprovided pattern, and/or with alignment marks provided on the substrate.

In this specification, the term “substrate” is meant to include anysurface layers forming part of the substrate, or being provided on asubstrate, such as other planarization layers or anti-reflection coatinglayers between the self-assembled polymer layer and the substrate.

The invention claimed is:
 1. A method of lithography on a surface of asubstrate having a self-assembled polymer layer thereon comprising firstand second chemically-distinct domains arranged in a pattern across thelayer, the method comprising: forming a planarization layer over thelayer of self-assembled polymer, the planarization layer having a firstportion over the first domain and a second portion over the seconddomain; applying a development etch to the planarization layer tosubstantially remove the second portion leaving at least part of thefirst portion as a cap substantially covering the first domain to formcapped first domain; substantially removing the second domain from thesurface, leaving the capped first domain as a pattern feature on thesurface; and transferring the pattern feature to the substrate using thecapped first domain as an etch resist.
 2. The method of claim 1, whereinthe first domain has a first thickness less than a second thickness ofthe second domain.
 3. The method of claim 2, comprising applying a firstetch to the self-assembled polymer layer such that the first domain isetched to a first thickness less than a second thickness of the seconddomain prior to forming the planarization layer.
 4. The method of claim3, wherein the first etch is a directional plasma or photolytic etch. 5.The method of claim 4, wherein when the first etch is a directionalplasma etch and it is an oxygen plasma etch or an argon plasma etch. 6.The method of claim 3, wherein the second thickness is 5 nm, or more,greater than the first thickness.
 7. The method of claim 3, wherein thefirst thickness is 5 nm or more.
 8. The method of claim 1, wherein theplanarization layer is applied by spin-coating a liquid planarizationcomposition onto the self-assembled polymer layer and solidifying theliquid planarization composition to provide the planarization layer. 9.The method of claim 1, wherein the planarization layer comprises aplanarization material selected from the group consisting ofsilicon-containing acrylate polymer, hydrogen silsesquioxane andpolydimethylsiloxane.
 10. The method of claim 1, wherein substantiallyremoving the second domain from the surface is performed by a separateetch from an etch used in transferring the pattern feature to thesubstrate using the capped first domain as an etch resist.
 11. Themethod of claim 1, wherein the development etch is a separate etch froman etch used to substantially remove the second domain from the surface.12. A method for transferring a pattern feature from a self-assembledpolymer layer comprising first and second chemically-distinct domainsarranged in a pattern across the layer on a substrate, to the substrate,the method comprising: applying a first etch to the self-assembledpolymer layer such that the first domain is etched to a first thicknessless than a second thickness of the second domain; forming aplanarization layer over the layer of self-assembled polymer, theplanarization layer having a first portion over the first domain and asecond portion over the second domain; applying a development etch tothe planarization layer to substantially remove the second portionleaving at least part of the first portion as a cap substantiallycovering the first domain to form capped first domain; substantiallyremoving the second domain from the surface, leaving the capped firstdomain as a pattern feature on the surface; and transferring the patternfeature to the substrate using the capped first domain as an etchresist.
 13. The method of claim 12, wherein the second thickness is 5nm, or more, greater than the first thickness.
 14. The method of claim12, wherein the planarization layer comprises a planarization materialselected from the group consisting of silicon-containing acrylatepolymer, hydrogen silsesquioxane and polydimethylsiloxane.
 15. A methodof forming a pattern feature on a substrate from a self-assembledpolymer layer thereon, the self-assembled polymer layer comprising firstand second chemically distinct domains arranged in a pattern across thelayer, the method comprising: forming a planarization layer over thelayer of self-assembled polymer, the planarization layer having a firstportion over the first domain and a second portion over the seconddomain; applying a development etch to the planarization layer tosubstantially remove the second portion leaving at least part of thefirst portion as a cap substantially covering the first domain to formcapped first domain; and substantially removing the second domain fromthe surface, leaving the capped first domain as a pattern feature on thesurface.
 16. The method of claim 15, wherein the first domain has afirst thickness less than a second thickness of the second domain. 17.The method of claim 16, comprising applying a first etch to theself-assembled polymer layer such that the first domain is etched to afirst thickness less than a second thickness of the second domain priorto forming the planarization layer.
 18. The method of claim 17, whereinthe second thickness is 5 nm, or more, greater than the first thickness.19. The method of claim 17, wherein the first thickness is 5 nm or more.20. The method of claim 15, wherein the planarization layer comprises aplanarization material selected from the group consisting ofsilicon-containing acrylate polymer, hydrogen silsesquioxane andpolydimethylsiloxane.